Non-recursive composite subsets

[jsub1]

Fix for #1348. This PR adds a helper function that allows traversing the state tree in composite subset states without making recursive function calls.

[codecov]

Codecov Report

Merging #2140 into master will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2140      +/-   ##
==========================================
- Coverage   87.99%   87.86%   -0.13%     
==========================================
  Files         245      246       +1     
  Lines       22322    22685     +363     
==========================================
+ Hits        19642    19932     +290     
- Misses       2680     2753      +73     

Impacted Files	Coverage Δ
glue/core/subset.py	89.57% <100.00%> (+0.27%)	⬆️
glue/core/fixed_resolution_buffer.py	88.52% <0.00%> (-6.12%)	⬇️
glue/utils/array.py	89.49% <0.00%> (-5.86%)	⬇️
glue/core/component.py	91.28% <0.00%> (-5.78%)	⬇️
glue/viewers/image/python_export.py	86.53% <0.00%> (-4.64%)	⬇️
glue/viewers/matplotlib/state.py	90.18% <0.00%> (-3.94%)	⬇️
glue/viewers/matplotlib/layer_artist.py	95.55% <0.00%> (-1.89%)	⬇️
glue/viewers/matplotlib/viewer.py	93.92% <0.00%> (-1.36%)	⬇️
glue/core/data.py	90.22% <0.00%> (-0.90%)	⬇️
glue/viewers/profile/layer_artist.py	95.09% <0.00%> (-0.74%)	⬇️
... and 21 more

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[astrofrog]

I'll try reviewing this soon, but in the mean time, could you fix the code style issues and also add a regression test that would normally have failed due to a recursion error?